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Sulfur hexafluoride is pretty awesome. It is a gas six times denser than regular air, and thus you can actually make tinfoil "boats" float on it. It also makes your voice deep when you breathe it in.

Let's have a sea of it. To prevent the entire planet from becoming a barren wasteland, make it a single sea about the size of the Black Sea, with an island the size of Cyprus in the middle. It's surrounded by tall mountains so there's little wind that would blow it empty. When, and with what technology, could the first people reach the island?

Sulfur hexafluoride is dense, but not dense enough for a regular boat to float on. You could not walk to the island either, because the gas will displace oxygen in your lungs and you will asphyxiate.

Because it is so dense, I wonder if you could navigate this sea with some kind of boat/plane hybrid. Or something similar to Da Vinci's flying machine. Could people reach this island at any time before the invention of the hot air balloon?

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    $\begingroup$ I am assuming technology level on this question is pretty much "As low as you can get it"? $\endgroup$ – Plutian Feb 18 '20 at 15:58
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    $\begingroup$ @Plutian Pretty much. People would live near this sea, and would try to cross it from time to time, for the same reason people scaled Everest. When would they first be able to? $\endgroup$ – KeizerHarm Feb 18 '20 at 15:59
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An ordinary bag filled with ambient air, if large enough, will float a ship's deck. If you have large fins going down below the air bag, they can act as a keel to react against sails (though if you have enough wind to sail, you're in danger of losing your "airsea").

Basically this would be a gas balloon, only the lifting gas is air -- every cubic meter of air displaces six times its weight in SF6, so you can calculate how big the gas bag needs to be based on how heavy your "ship" is. If you build fairly light (say, largely wicker construction like balloon baskets), I'd expect the bag to be significantly smaller than the gas bag of a hydrogen balloon, as you'd get a bit more than five times the lift per bag volume.

To put numbers on this, air at sea level pressure and room temperature is just over 1.2 kg per cubic meter, so each cubic meter of bag can support roundly 6 kg of bag skin, structure, and payload. A simple craft built like a basket with outriggers (to spread the lift for stability) might weigh as little as a couple hundred kg -- plus "airsailor(s)" and whatever propulsion method, and provisions, call it 500 kg for the first one, that would only need 80-90 cubic meters of air.

How early (technologically) could this happen? How early would someone notice a waterskin floating on the "dead air?"

Another answer mentioned propulsion -- as I noted, sailing won't work, if the sea is to stay in place. However, with no appreciable friction from the "airsea", you don't need the kind of power you'd need on water. A flapping, flexible fin (like a single wing mounted upright) would provide enough propulsion to get across, slowly -- a slow walking pace.

The biggest hazard to this, once you have a large enough bag and a design that's stable, is that the air and "dead air" will mix a bit at the interface, so your "airsailors" will be in danger of suffocation unless their craft holds them a few meters above the top of the air bag.

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Problem 1: Weight
The weight is the largest issue here. If you want any kind of boat to float on this while supporting humans, you will need something extremely light, and significantly large enough to carry a human. This could be anything from large drums containing air to create a raft, but a fair few of them would be needed. Or a large enough boat structure to carry a human. In any case, light woods like balsa would be your friend here.

Problem 2: Propulsion
If you state the wind won't blow off the gas, sufficient to say there is no wind to speak of. This basically kills the idea of sailing across, and a different method of propulsion is needed. Rowing is unlikely to do you much good, and with the average depth of the black sea being almost a mile, a pole pushing from the bottom is unachievable. Perhaps this can be solved by large flapping "wings" on your craft, or a bellows based device, but this likely won't be very effective. In this case being pulled along by a flock of trained pigeons doesn't seem so ridiculous all of a sudden.

My solution:
Probably not the best, but feasible nonetheless. Get a regular air balloon made of leather or any air-tight material. Attach to that an insanely long tube made of the same material, or something like animal intestine. Use the tube to breathe while the balloon floats the ends on top of the sea, and simply walk across. Optional backpack mounted hand powered air pup device, and a cart to carry unused tube. Gas pressure would get to you a bit, but won't nearly be as harsh as water pressure at this depth.
Problem:
It is too long a walk to do in one go. So an option is to build rest stops on the bottom of the sea, using similar breathing technology. Note: I haven't crunched the numbers, it could be this solution is utterly impossible due to pressure.

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  • $\begingroup$ Your tube will collapse from the gas pressure at only a few meters of depth, if it's made from anything like intestine; it'll be impossible to inhale. Not to mention that if the tube is more than a couple meters long, you'll need to have separate inhalation and exhalation tubes or suffocate in your own breath. $\endgroup$ – Zeiss Ikon Feb 18 '20 at 16:54
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    $\begingroup$ @ZeissIkon Yes, I considered this, but perhaps filling the tube (sparingly) with a springy straw like material would give enough outwards pressure to prevent it collapsing, while leaving enough space for air to pass. I thought of the two tubes, but you don't actually need to do anything with your used breath, just don't breath out into your tube. $\endgroup$ – Plutian Feb 18 '20 at 16:59
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Your travelers could use a hot air balloon -- which has nothing to do with the sea. That won't happen until later in their civilization, though- but it is the earliest example of a flying machine.

That being said, the first people to reach the island may have used a scuba-type device . A Sulfur hexafluoride diver would carry what appears to others as a balloon that they're sucking air out of.

We typically cannot dive very deep in water while breathing atmospheric air because the weight of the water is to great to actually move your chest to aspirate. That's why pressurization is important to divers. Luckily, we're not dealing with water.

The black sea is about 7,000' deep . . . With six times pressure on that, it would be equivalent to trying to breathe while 42,000 feet below sea level. I wasn't able to find much on breathing while 8 miles below sea level, but if you keep the air supply at the same depth as your diver, my understanding is that the pressure on the air bladder will help "push" the air into your lungs.

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  • $\begingroup$ An air bag at ambient pressure is good for only minutes, at most. You inhale around 2 liters per breath; how many breaths does your bag hold? $\endgroup$ – Zeiss Ikon Feb 18 '20 at 16:55
  • $\begingroup$ My hypothetical bag was arbitrarily large. $\endgroup$ – Aww_Geez Feb 18 '20 at 17:06
  • $\begingroup$ At which point, it becomes my bag, and you'll be floating on/under it, not just breathing from it. Remember, about five times the lifting power of hydrogen in air. $\endgroup$ – Zeiss Ikon Feb 18 '20 at 17:07
  • $\begingroup$ I've added numbers to my answer. Looks like a primitive, early craft would need 80-90 cubic meters of air to float roundly 500 kg lf structure, bag skins, and payload. That could be as simple as 150 steer hides, or 240 hog skins, retained in basketry. $\endgroup$ – Zeiss Ikon Feb 18 '20 at 17:26
  • $\begingroup$ Suppose we could build a device at a very lean 200lbs (including passenger) then we'd need to displace 520 cubic feet of sea-gas to float it. (2.6 cubic feet of Sulfur hexafluoride = 1lb) $\endgroup$ – Aww_Geez Feb 18 '20 at 17:27

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